Process of making vitreous enamel coated sheets



[PAT NT OFFICE PROCESS oF MAKING VITREOUS ENAME I .coA'rsn' SHEETS 5 Joseph C. EckeLIngram, Pa.

No Drawing.

This invention relates to a method of processing low carbon sheet steels adapted to be 'subse-. quentlycoated with vitreous enamel andhas for its object the provision of sheet steel which may be coated I with vitreous enamel without re boiling. 7

Re-boiling, as used in referring to sheet steels having a vitreous enamel coating, is a term'used to describe enamel defects in the form of minor bubbles or blisters in the ground coat formed during the firing or heating of the cover coat enamels. The most serious of such enamel defects are termed black spots. This major de- I fect which together with other defects known as r step of heavy cold fish scales and copper heads at times result from improperly processed steel which is subsequently coated with vitreous enamels. Black spots are dark round discolorationsappearing in the finish or cover coat enamel. Fish scaling is the term used when referring to enameled sheets from which small chips of substantially half moon shape have broken. In some cases these chips are so small that they cannot be seen with a magnifying glass while in other cases they may measure a full one-quarter inch in diameter. Copper heads is a termed used to describe a defect which appears in a finished enamel sheet as a brownish red spot or as a depressed red spot.

I have discovered that re-boiling of enamel, causing black spots, fish scaling and copper heads are directly attributable to the re-boiling characteristic of the base metal produced under any method other than that disclosed and claimed herein. However, it is readily understood by those skilled in the art of enameling that the same enamel defects above mentioned can be obtained by improper enameling practice regardless of the character or quality of the steel base.

I have further discovered that these above mentioned defects of enameled sheets can be prevented if the steel is of a low carbon type (pref-v erably having a carbon content of less than .12 per cent) and so processed as to include a final reduction.

By a heavy cold reduction is meant the reduction in gage thickness of not less than about thirty per cent and not more than approximately ninety per cent (90%). Preferably it is between about fifty to'sixty per cent to of the average gage of sheet metal as used for enameling purposes.

These severely reduced sheets may be heat treated or otherwise mechanically processed prior to or following said cold reduction in order to present ductility and flatness to the material. The cold reduction step of manufacturing sheets Application November 21, 1933, Serial lio.-699,091 1 2 Claims. 1(01. 91-73) in accordance with this invention may be carried out on any standard form of cold reducing mill and this cold reduction may be applied to sheets in any form whether in multiple lengths such as strips or in single sheets or packs of sheets.

A sheet, as the term is used throughout this specification and claims, is meant to include any sheet or'strip of metal produced for vitreous enamel coating.

After the sheets have been produced as described above they are subjected to the usual ground coating of vitreous enamel which is fused thereon by firing or heating in a suitable furnace to a temperature of from 1550 degrees F. to 1625 degrees F. and then permitted to cool to room temperature. After the ground coat of vitreous enamel has cooled one or more final cover coats of'vitreous enamel are applied, and after each coat is applied the sheet is preferably heated or fired to burn the cover coats on the metal.

If re-boiling, black spots, fish scaling or copper heads appear'on the finished enameled coating, following such a typical enameling process as just described, these sheets have no commercial value. The'quality of the sheets produced by my method eliminates the formation of these defects and provides a smooth, even,- tight layer of cover coat enamel.

While I have described the certain specific embodiments of my invention, it is to be understood that I do not wish to be limited thereto since various modifications may be made without departing from the scope thereof, such as defined in the appended claims.

I claim:

1. The method of producing a vitreous enameled coated steel sheet having a quality and surface free from'boiling defects which consists in reducing a sheet of low carbon steel an amount so as to leave at least a 30 per cent reduction necessary to produce the finished gage, then reducing the sheet to finished gage by cold rolling, and finally applying and burning the vitreous enamel coating thereon.

2. The method of producing a vitreous enameled coated steel sheet having a quality and surface free from boiling defects which consists in reducing a sheet of steel having less than .12 carbon, an amount so as to leave from 50 to per cent reduction necessary to produce the finished gage, reducing the said sheet to finished gage by cold rolling, subjecting said sheet to a suitable thermal process to produce ductility, and finally applying and burning the vitreous enameled coating thereon.

JOSEPH C. ECKEL. 

